1. Field of the Invention
The invention relates to optical fiber couplers made by fusing and tapering the optical fibers and fabrication of such couplers so as to provide multiplexing and demultiplexing optical functions with minimal polarization effect. This invention also relates to the design of such couplers having predetermined wavelength periods.
2. Description of the Prior Art
Fused tapered couplers are made by laterally fusing and tapering two or more optical fibers. The technique enables an exchange of light power between two or more optical fibers and can be used to fabricate power splitters. One advantage of this method is that light never leaves the glass of the optical fiber and never encounters an interface, making the coupling process inherently reflection free.
Initially, this technique was discovered at the Canadian government communications research centre in Ottawa with reference to multimode optical fiber distribution systems, which lead to several patents, such as U.S. Pat. Nos. 4,291,940; 4,330,170; 4,439,221; 4,449,781; 4,586,784 and 4,763,977. It was soon realized that it also worked with monomode or single-mode fibers as disclosed, for example, in U.S. Pat. No. 5,054,874, but with a somewhat different behaviour.
With single-mode fibers, the coupling of light between the fibers was oscillatory, as a function of elongation, and thus the coupling ratio could be controlled. Furthermore, it was also observed that this behaviour was oscillatory in wavelength and thus the couplers could be used as wavelength multiplexers and demultiplexers as disclosed, for example, by Bures et al., Applied Optics, 1983, 22(12). In the telecommunications domain, the realization of multiplexers was published, for instance, by Lawson et al., 1984, Electronics Leters, 20(23). It was then determined that the period could be controlled by the number of coupling cycles which are observed during the elongation process. In the early 1980""s, the only commercially available multiplexing fused couplers were of large periods (1300 nm-1550 nm) corresponding to 1.5 or 2 elongation cycles. However, it was later shown by Bilodeau et al., Optics Letters, 1987, 12(8), that couplers with large number of cycles had much smaller periods than those with a small number of cycles. The experimental wavelength response of long couplers shows a beating phenomenon, where the sinusoidal spectal response is modulated. This is explained by the modulation attibued to the slight difference in modal propagation constants, as disclosed, for example, by Love et al., Electronics Letters, 1985, 21(12). It became obvious then that to make a good multiplexer with a small wavelength spacing, one had to make a long coupler with many cycles and that for the multiplexed wavelengths, the two polarization states should be in phase, as shown, for instance, by McLandrich et al., Journal of Lightwave Technology, 1991, 9(4).
There are also patents that describe this principle. For example, U.S. Pat. No. 5,491,764 discloses a narrow band twisted optical fiber wavelength division multiplexer/demultiplexer (WDM) where a pair of fibers is first twisted to reduce polarization dependence and then fused to form a coupler. It is stated in this patent that although there exist fiber optic WDMs that use optical fibers which are aligned in parallel with one another and fused to form a fiber optic coupler, they are only capable of MUXing and DEMUXing two preselected wavelength lights, operating at wavelengths of 1310 nm and 1550 nm.
In U.S. Pat. No. 5,809,190 there is disclosed a multi-window wavelength-division multiplexer (MWDM) in which two fibers are crossed and fused together to form a multiplexer coupler. It is stated in this patent that it uses a crossed pair of fibers, instead of a prior art twisted pair of fibers, to improve the polarization dependent loss. By reducing the polarization sensitivity, U.S. Pat. No. 5,809,190 indicates that more than two wavelengths can be multiplexed which is obvious for sinusoidal wavelength response because such response is periodic. This principle was disclosed by Symon et al. in a paper entitled xe2x80x9cDense all fiber WDM by means of Mach-Zehnder interferometerxe2x80x9d presented at SPIE Photonics West ""96 conference on Functional Photonic and Fiber Devices, held in San Jose, Calif. on Jan. 28-Feb. 2, 1996 and published in the SPIE Proceedings Vol. 2695 pp. 114-122.
Neither of the above patents describes ways to achieve the correct spacing and to match the polarization phase siunultaneoasly, for any given channel spacing. Therefore, there is a need for multiplexing and demultiplexing couplers with narrow channel spacing, wherein one would simultaneously obtain a predetermined wavelength spacing and the required polarization phase match.
The present invention provides a method of fabrication of multiplexing and demultiplexing couplers with narrow channel spacing of 0.4 nm or larger by controlling the degree of fusion and the shape of the longitudinal profile of the fused fibers. This can be done without either twisting or crossing the single-mode fibers from which the couplers are made by fusion and elongation. This allows a more precise control of the response of the coupler and makes it possible to achieve a match between spacing and polarization for any given channel spacing, that will be reproducible in fabrication. This is possible because the control can be used to reduce or increase polarization dependence and wavelength dependence so that the match can be made for any desired condition. The invention also includes the novel couplers produced pursuant to the new fabrication process.
The principle of operation of single-mode fused fiber couplers is now well known. For simplicity, we will only describe the operation of a 2xc3x972 coupler, i.e., a coupler composed of 2 fused single-mode identical fibers. Although the basic principle presented here is applicable to other fused structures, using more than 2 fibers or dissimilar fibers, most of the discussion herein is oriented towards making a 4 port-device, i.e., 2 input ports and 2 output ports, that can multiplex or demultiplex two series of wavelengths.
In making a 2xc3x972 single-mode fused fiber coupler, two optical fibers are placed side-by-side after stripping of the protective polymer jacket, so that the optical claddings of the fibers are longitudinally in contact over a predetermined length. Such contact can be mechanically maintained or, as indicated in some prior art references mentioned above, the fibers can be crossed or twisted together. The exposed section is placed between two holding clamps that suspend it so that a heat source can be approached to fuse and soften the glass, and to create taper by pulling on the clamps. This creates a bi-taper structure, made of two fibers that share a single optical cladding because they are fused together. If the taper transverse dimensions are small enough, the fiber cores are reduced to a point where they do not guide the light anymore. This power is then guided by the optical cladding and the surrounding medium, which is usually air, thus forming a highly multimode waveguide.
Because of the transverse symmetry of the structure, composed of two fused identical fibers, the single-mode fiber core mode excites, in the down-taper region, a superposition of two optical modes of the fused and tapered region. These modes, hereafter called supermodes, are the fundamental mode, labelled LP01 and the first asymmetric mode, labelled LP11. If the transition in the down taper region is adiabatic. i.e. the taper slope is not too abrupt, the two supermodes are exited equally and no power is lost to higher power modes. The two supermodes then propagate along the fused section, accumulating a phase difference xcfx86. In such adiabatic up-taper region, the supermodes interfere and the power goes back into the fiber cores. Depending on the phase however, the interference will be either constructive in the initial fiber core or if the modes are out of phase, in the secondary fiber core, thus transferring the power from one fiber to the other. The transmission of a coupler of length L can be described by a transfer matrix T(xcfx86),       (                                                      a              1                        ⁡                          (              L              )                                                                                      a              2                        ⁡                          (              L              )                                            )    =                    T        ⁡                  (          ϕ          )                    ⁢              (                                                                              a                  1                                ⁡                                  (                  0                  )                                                                                                                          a                  2                                ⁡                                  (                  0                  )                                                                    )              =                            ⅇ                      t            ⁢                          xe2x80x83                        ⁢                          ϕ              _                                      ⁡                  (                                                                      cos                  ⁢                                      xe2x80x83                                    ⁢                  ϕ                                                                              ⅈ                  ⁢                                      xe2x80x83                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                  ϕ                                                                                                      ⅈ                  ⁢                                      xe2x80x83                                    ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                  ϕ                                                                              cos                  ⁢                                      xe2x80x83                                    ⁢                  ϕ                                                              )                    ⁢              (                                                                              a                  1                                ⁡                                  (                  0                  )                                                                                                                          a                  2                                ⁡                                  (                  0                  )                                                                    )            
where xcex11 and xcex12 are the optical amplitudes in the first and second fiber respectively at the input of the coupler (xcex11(0)) and at the output of the coupler (xcex11(L)), where the accumulated phases are defined as       ϕ    =                  ∫        0        L            ⁢                                                  B              1                        -                          B              2                                2                ⁢                  ⅆ          z                      and            ϕ      _        =                  ∫        0        L            ⁢                                                  B              1                        +                          B              2                                2                ⁢                  ⅆ          z                    
The phases are integral over the length L (along the propagation axis z) because the longitudinal profile of a tapered coupler varies along L, i.e., the transverse dimensions are tapered down then up, and the propagation constant of the supermodes B1 and B6 for the supermode LP01 and the supermode LP11 respectively depends on the local transverse dimension.
This transmission of a coupler, for an input in one of the branches (xcex11(0)=1, xcex12(0)=0) is thus given by
P1=cos2(xcfx86)
P2=sin2(xcfx86)
where P1 and P2 is the output optical power in the first fiber and second fiber respectively.
The oscillatory behaviour of a 2xc3x972 coupler thus depends only on the accumulated phase difference xcfx86. When one elongates a coupler, one obverses an oscillatory behaviour. This is due to the phase difference xcfx86 being increased as the length L of the coupler increases. When a coupler is measured in wavelength, one can also notice this oscillatory behaviour, because the phase difference xcfx86 also increases approximately linearly with wavelength. Furthermore, when couplers are elongated several cycles, a beating phenomenon is observed in the cycles if an unpolarized light source is used. This effect is due to the birefringence of the coupler. One can define two orthogonal axes of polarization, which are the two axes of symmetry of the coupler, defined here as x-axis and y-axis. For each as, there are 2 supermodes, LP01x, LP11x, and LP01y, LP11y. If each polarization is independent of the other, i.e., if they are not coupled in the coupler, the transmission can be written for each polarization as follows:
P1x=cos2(xcfx86x)
P1x=sin2(xcfx86)
P1y=cos2(xcfx86y)
P2y=sin2(xcfx86y)
where xcfx86x and xcfx86y are the accumulated phase differences of the two polarization states. The total output power for all states of polarization can thus be written as follows.
P1=(xcex11x(0))2P1x(xcfx86x)+(xcex11y(0))2P1y(xcfx86y)
P2=(xcex12x(0))2P2x(xcfx86x)+(xcex12y(0))2P1y(xcfx86y)
where xcex11x(0) and xcex11y(0) are the amplitudes of each polarization state at the input of the coupler. For normalized power input, (xcex11x(0))2+(xcex11y(0))2=1.
The resulting output is thus a modulated sinusoidal response, the modulation amplitude being determined by the ratio of the initial polarization amplitude. When monitored with an unpolarized light source, the modulation amplitude is maximum because both states of polarization are excited with equal amplitude. The transmission thus becomes represented as follows:
P=1/2(P1x(xcfx86x)+P1y(xcfx86y))
P2=1/2(P2x(xcfx86x)+P2y(xcfx86y))
As a function of elongation or wavelength, the measured response of a coupler with an unpolarized source will show a rapid power oscillation between the two output ports, the amplitude of which is modulated, i.e., for which the contrast varies from 0 to 1. In this modulated oscillation, when the amplitude is maximum, i.e., when there is a complete power exchange between the two ports, the polarization phases xcfx86x and xcfx86y are matched, i.e. their difference is a multiple of 2xcfx80. When the contrast is 0, i.e., when the rapid oscillation amplitude almost disappears, the power is divided 50% /50% between the ports, and the polarization phases are out of phase by xcfx80. Therefore, to make a good multiplexer/demultiplexer, one has to match both polarization phases, so that the amplitude of the power exchange is maximum whatever the state of polarization.
According to this invention, in order to match the two polarization states at a given wavelength spacing, the parameters that change the fused fiber transverse and longitudinal shape must be controlled. These parameters relate to the local cross-sections of the fused fibers and include the degree of fusion between the fibers and the reduction ratio defined as the reduced cross section dimension/initial dimension before tapering. Their value varies with the tapered structure, i.e. with the longitudinal profile, including both the variation of degree of fusion and of reduction ratio.
Accordingly, one of the objects of this invention is to control the phase of the two polarization states. However, such control would be difficult if there was a coupling between the polarization states. Thus, it is a preferred feature of this invention to keep the fibers of the coupler in parallel alignment, i.e. untwisted and uncrossed, during fusing. In such a fused fiber coupler, the two polarization states do not couple and therefore provide no uncertainty in the process arising due to coupling that may occur with twisted or crossed fibers. Furthermore, it is difficult to measure parameters such as the degree of fusion, when the fibers are twisted or crossed, making the control of such parameters more difficult. Nonetheless, the invention may also be applied to couplers with twisted or crossed fibers, although with greater difficulty of control.
It is also an object of this invention to teach how to change the degree of fusion and longitudinal profile in order to obtain the matching of the polarization phases at a given wavelength separation.
It is a further object of this invention to provide a matching which does not result in a unique profile, but rather several profiles may produce such a matching because the process is periodic in nature. The choice of the matching point will depend on the restriction in length that might be imposed on the coupler design due to a packaging size limitation, or the desire to me the polarization effect for more than two wavelengths.
In order to understand how to achieve the polarization phase matching, one has to understand how the wavelength response of a coupler is influenced by its degree of fusion and reduction ratios.
The degree of fusion is a measure of the shape of the cross-section of the fused coupler. By definition, it varies from 0, when the fibers are barely touching each other to 1 when the two fibers are completely fused, making the cladding of the fused structure cylindrical in form. There is a direct relation between the degree of fusion and the distance between the fiber cores, which are closest when the degree of fusion is 1.
The reduction ratio is the measure of the taper profile. It is the scaling factor of the cross-section as the taper is made. It is assumed that when the fibers are tapered, the reduction in size is always proportional for both x and y axes.
Both parameters will influence the local difference of the supermodes propagation constants xcex94xcex2=B1xe2x88x92B2. At a given wavelength, for a given degree of fusion, xcex94xcex2 will increase as the taper size is reduced. For a given reduction ratio, xcex1xcex2 will also increase if the degree of fusion is increased.
The effect of these parameters on the wavelength dependence is more complex. First, for a given degree of fusion and reduction ratio, xcex94xcex2 increases with wavelength. Thus, the accumulated phase xcfx86 of a coupler of length L also increases with wavelength, thus giving the coupler its oscillatory wavelength response. However, the wavelength period will depend on the slope dxcfx86/dxcex of the phase. Thus, for a given degree of fusion and reduction ratio, an increase in the length L will reduce the wavelength period. That is why longer couplers have narrower wavelength response. Furthermore, for a given length L and a given degree of fusion, a coupler with a smaller cross-section (a smaller reduction ratio) will also have a smaller wavelength period. However, for a given length and a given reduction ratio, a greater degree of fusion will increase the wavelength period.
Thus, all these parameters influence the wavelength period of a coupler. The actual coupler is even more complex because the phase is an integral over the longitudinal profile of length L of all the different xcex94xcex2 given by the local fusions and reduction ratios. Because of this, however, it is possible to modify the profile of a coupler to change its properties.
For example, when a coupler is made, one can monitor the elongation oscillatory response. When monitoring at wavelength xcex1, the response is periodically maximum. If one stops at the Nth maximum, and looks at the wavelength spectrum. One will see a given wavelength period, thus creating a multiplexer between xcex1 and xcex2, with a channel spacing of xcex4xcex2=xcex1xe2x88x92xcex2. If one continues the elongation to the next maximum at xcex1, one will increase the phase, and thus decrease the wavelength period, thereby creating a multiplexer between xcex1 and xcex3 with a channel spacing of xcex4xcex3=xcex1xe2x88x92xcex3 less than xcex4xcex2. This allows to make a multiplexer with different spacings. However this spacing, at a given wavelength, is discrete. Varying the degree of fusion or the longitudinal profile will help change these discrete points so as to match the spacing with the maximum at a given wavelength. For example, if one wants xcex4xcex2 greater than xcex4xcex2.  greater than xcex4xcex3, one can either increase the size of the cross-section by using a wider heat source or by increasing the fusion This will shift xcex4xcex2 towards 67 xcex2. Or if one decreases the size of the cross-section or decreases the fusion, one will shift xcex4xcex3 towards xcex4xcex2. These two parameters can be controlled in a continuous fashion, thus making it possible to match any wavelength period at any wavelength.
According to this invention, it is possible to use the same principle as explained above to match the wavelength periods and the polarization phase at the same time, since the same reasoning can be applied to the polarization phase matching.
Thus, it is first necessary to look at the influence of the fusion and ratio parameters on the polarization phases. Except in the case of extremely lightly fused couplers, the two polarization phases are almost equal for couplers with full cross-section dimensions and their difference increases as these dimensions get smaller. However, this dependence is not proportional to the degree of fusion, being largest for a degree of fusion close to 0, minimum at a value between 0.4 and 0.7, and slightly larger for a degree of fusion of 1.
As a function of the reduction ratio, the smaller the coupler, the greater the difference between the two polarization phases. And the increase is exponential with the size. This is due to the supermode fields which are larger at the cladding-air interface and the large index step has a large influence on the x- and y-polarization difference.
As with the supermode phases xcfx86x and xcfx86y, the polarization phase difference xcfx86xy also accumulates a phase difference along the coupler. Because both are positive and of a small order of magnitude, the phase difference xcfx86xy is smaller than either xcfx86x or xcfx86xy. That essentially means that, as a function of elongation when monitored with an unpolarized light source, the coupler wall go through many power exchange cycles before xcfx86xy is equal to xcfx80, which corresponds a null point in the contrast or 2xcfx80 for the first maximum contrast point, i.e. the polarization phase matching point. It is close to the points where xcfx86xy is a multiple of 2xcfx80, that the multiplexing coupler should operate, the polarization beating phenomenon being also a function of wavelength.
Thus, when a coupler is elongated, power cycles can be monitored and elongation can be stopped when the first polarization phase xcfx86xy match point is reached. Then, if measured as a function of wavelength, one will observe a maximum contrast at the monitoring wavelength, contrast which will decrease as one looks further away from the monitoring wavelength. If one looks at the difference between the first maximum and the first minimum on either side of the monitoring wavelength, one will get the channel spacing of this particular multiplexing coupler. If the elongation is continued to the next polarization phase match point, the wavelength period will be smaller One can continue elongation to the next phase matching point to obtain again a smaller period, and so on. As with the multiplexer phase itself, these points are discrete points that have specific wavelength periods. If these periods do not correspond to the desired period, one can adjust the phase wavelength slope dxcfx86/dxcex so that the phase matching point also matches the desired period. One can adjust the fusion or the longitudinal profile to achieve this match.
If the phase matching point period of the closest phase matching point is smaller than the desired period, it means that the coupler is not polarization dependent enough. This can be corrected by creating a profile with a smaller waist which will increase the polarization dependence. The phase match point will occur sooner in elongation and thus will be smaller, while the corresponding period will be larger. With a good control of the coupler parameter, it can be made to match the desired period. When the period is so matched, a small adjustment in the length may be needed so as to match the maximum and minimum with the appropriate wavelength. When the number of cycles is large, the first polarization phase match point occurs only after many cycles, and the period does not significantly change with a shift of a fraction of the period; thus the adjustment can be made without changing the period. This is not true, however, for a coupler with only a few cycles, where the profile has to be changed in order to make the period and the wavelength match. Inversely, if the phase matching point period of the closest phase matching point is larger than the desired period, then one has to make the profile waist larger to reduce the polarization dependence. This will make the coupler length longer, but it will be possible to phase match both the polarization and the period.
It is important to note that the phase match can be realized by changing the degree of fusion rather than the profile, or a combination of both. The effect of changing the degree of fusion can be significant. For a given profile, it is possible to make the first polarization match point period using a degree of fusion of 0.4 equal to the second polarization match point period of a coupler that has a 0.1 degree of fusion.
Thus, according to this invention, it is possible to control the polarization phase match point period of the couplers by controlling both the degree of fusion, i.e. the cross-section of the coupler, and the shape of the longitudinal profile, by varying the heat source position and/or shape and the speed of elongation.
Because of the sensitivity of both the fusion parameter and the longitudinal profile, and because the method to obtain the appropriate wavelengths and periods is iterative, i.e., a coupler has to be made, measured and the fabrication parameters have to be modified to make the next attempt closer to the goal, the fabrication process must be repeatable. This is why it is necessary to separate the steps of fusing the fibers and elongating the structure, so that the degree of fusion may be made reproducibly and measured.
It is thus part of the process of this invention to fuse the coupler in a separate step before proceeding with the elongation and tapering of the fused fibers. Furthermore, the fusion step can be realized with a different heat source than the elongation and tapering step. To decrease the time of fusion, another torch tip, creating a hotter flame, can be used to fuse the fibers. Also, the flame is preferably approached from the side, so that the gas flow from the torch tends to push the fibers together, facilitating the fusion process. The heat source is swept along the fusion region, spending more time in the middle of the fused region, to create a gradual variation in the degree of fusion so that no loss is induced in the optical power transmission during and after fusion and during and after tapering.
It is a feature of the method of this invention to achieve the polarization phase matching period by iteratively adjusting the longitudinal profile and the degree of fusion. This is realized by changing the heat source position and/or shape and the speed of elongation. Once the general properties of a multiplexer are thus achieved, one can go to more detailed optimization.
The important parameters of the multiplexers and demultiplexers will now be described in greater detail.
A multiplexing coupler has 3 ports that are used. There are two input ports where the different wavelengths are inserted and combine in the output port. The important optical parameters describing a multiplexing coupler are the insertion loss of each wavelength going through the devices. Ideally, the insertion loss should be 0 dB (corresponding to a normalized transmission of 1, i.e., a lossless device). For a demultiplexing coupler (which is exactly the same thing as a multiplexing coupler but used in reverse), the combined wavelengths are inserted in the single input port and are separated in each of the two output ports. In this case an additional important parameter is the isolation of the wavelengths, meaning the amount of power at a given wavelength, which is present in the other port with the other wavelength. The isolation should be as large as possible because the different wavelengths and thus the different signals will otherwise interfere at the output detector and thus cause errors, and any wavelength which is not in the appropriate port is lost for the transmitted signal. In both the multiplexer and demultiplexer, one other important parameter is the passband, i.e., the wavelength band around the desired wavelengths within which the device keeps certain properties, i.e., a given insertion loss or isolation. The simplest application of a multiplexing coupler is to multiplex or demultiplex two wavelengths.
In a polarization phase cycle, the polarization phase match point occurs at one wavelength xcexp. It is only at this wavelength in the polarization cycle that the transmission of the coupler is independent of polarization. The phase mismatch increases as one moves away from this wavelength. However, even with a mismatch, there is a point in each transmission halfcycle, where the output power is polarization independent because, at the transmission extremum of an unpolarized wavelength response Px=Py. The coupling ratio of this minimum dependence does however depend on the local value of the phase mismatch, and if this phase mismatch is large, the local cost will not be good. To minimize the polarization effect, one has to design the profile so that xcex1 less than xcexp less than xcex2, xcex1 and xcex2 being the two wavelengths to be multiplexed. Ideally xcexp=(xcex1+xcex2)/2. Being close to the polarization phase match point will insure a good isolation at both wavelengths. However, if one goes further away from this point, i.e., looking at the second, third, etc. extremum, the isolation will decrease and the insertion loss will increase. Though the multiplexer or demultiplexer may function, it will not be the optimum situation. The condition where the polarization phase match point is exactly in the middle of the two multiplexed wavelengths corresponds to a symmetry in the wavelength signal when, at both multiplexed wavelengths, the isolation and insertion loss are the same. If the xcex1=xcexp, the isolation would be greatest at that wavelength and less at xcex2, because the polarization phase mismatch is larger.
To achieve this condition, one can carefully tailor the coupler cross-section and longitudinal profile, but it is difficult to always match the polarization phase match point and the mid-point wavelength of the wavelength period. It was found, and it is a feature of this invention, that whereas heating and elongating the coupler increases all phases xcfx86x, xcfx86y, and xcfx86xy, making a small mechanical elongation without a heat source, herein referred to as xe2x80x9ccold-pullxe2x80x9d, increases the polarization phases xcfx86x and xcfx86y, but decreases, xcfx86xy. This effect can be used to opt the phase match in order to obtain a symmetrical isolation response between the two multiplexed wavelengths.
The precision in the centering of the channel can be obtained by small pulling adjustments which consist simply in stopping the elongation process and removing the heat source, at a point where the exact period and wavelengths are not yet reached, then monitoring the wavelength response. Thereafter, one reheats and pulls slightly in a very controlled manner, for a controlled time or distance and immediately removes the heat source. This will cause the coupler wavelength response to evolve in a very gradual and controlled manner, making it possible to target very precisely the wavelengths. Because the polarization phase effect of the cold pull can be very well calibrated, it is always possible to slightly overshoot the polarization phase match point and then bring it back with a cold-pull, so as to simultaneously match the multiplexed wavelengths with symmetry extremums in isolation.
It is part of the method of this invention to adjust the final coupler properties using the short heated controlled pulls and the final cold pull.
With such good control tools within the fabrication process, the design becomes very flexible and several applications can be optimized, such as applications involving several wavelengths, applications involving very narrow spacing (e.g. 1.6 nm channel spacing) and applications involving large wavelength spacing (30 nm to 70 nm).
As mentioned above, one can use more than one extremum away from the polarization phase matching point, but in such cases the performance may not be optimum. It may however by acceptable, so that the coupler could be used for more than two wavelengths, if such wavelengths stand on an approximately equally spaced grid. The optimization is similar as for the two wavelength multiplexer, except that the polarization phase match wavelength must be placed at xcexp=(xcex1+xcexn)/2, xcexp being the longest wavelength of the multiplexed series with xcex1 being the shortest. In this scheme the odd numbered wavelengths are multiplexed to the even numbered wavelengths. In the demultiplexer, a series of wavelengths separated by xcex4xcex will be demultiplexed into two series of wavelengths separated by 2 xcex4xcex. The centering technique is identical as for the two wavelength multiplexer. However, if one wants to optimize the performance of the device, one must try to reduce the polarization dependence so as to decrease the polarization phase mismatch within the wavelength range xcex1 to xcexn. This can be done by using a strong degree of fusion and a large cross-section profile. In this case, the polarization phase match occurs at a small number of polarization cycles, which makes the polarization phase difference slope smaller. This, however, will make the coupler physically longer. Thus, the method for producing such coupler is limited by the maximum length admissible for the packaging of the device.
It is, therefore, a feature of this invention to achieve a multi-wavelength operation of the multiplexers and demultiplexers by properly centering the polarization phase match point and reducing the polarization sensitivity by the control of the fusion and elongation profile of the coupler. This multi-wavelength operation is possible for a very narrow spacing (1.6 nm), but is more limited by polarization than for larger spacing because the polarization phase slope is much larger with the narrow spacing. For a large spacing (e.g. 10 nm) operation, over 8 wavelengths is possible as opposed to 4 for 1.6 nm spacing, with the same isolation criteria.
For very small channel spacing such as 1.6 nm, the number of power cycles is very large ( greater than 400) and the number of polarization cycles is also large ( greater than 30). This means that the period at each polarization cycle changes just a little. It is thus easier to achieve the proper period match than for larger periods, because of the discrete step in the period of the polarization cycles. Because of the adjustment possible with the fusion parameter and the longitudinal profile, it is possible to match the polarization phase of any period. This applies for any period that can be achieved after the first polarization phase match point in elongation. Depending on the profile, the channel separation is usually between 25 and 10 nm at this first polarization phase match point. It is thus possible to easily match any channel spacing, from 30 nm to 1 nm, by increasing or decreasing the polarization dependence using the degree of fusion parameter or the profile parameter. However, for spacing of about 30 nm, this is more difficult because the first polarization phase match point has not been reached during elongation and the zero point can not be moved. To make multiplexers with wavelength spacing of 30 nm and above, one must either greatly increase the polarization sensitivity or greatly decrease it.
For spacings above 60 nm, the solution, which is part of this invention, is to decrease the polarization sensitivity by using a strong fusion and a very long longitudinal profile. This solution does not place a polarization match point between the target wavelengths, but it minimizes the polarization mismatch. One could extend this solution to spacing below 60 nm, but the coupler usually becomes to long for it to be practical. The other solution is to make the coupler very polarization sensitive by moving the first polarization phase match point period up to the desired value between 30 and 60 nm. This requires a very small degree of fusion and an abrupt longitudinal profile. Because this produces an increase in polarization dependence, it will limit the operation of the device in a multi-wavelength configuration.
Thus, it is a feature of this invention to realize large period multiplexers using the long profile approach for larger spacings and using a profile with small fusion and size to realise raid-spacing multiplexers.